## CryptoDB

### Dakshita Khurana

#### Publications

**Year**

**Venue**

**Title**

2021

CRYPTO

One-Way Functions Imply Secure Computation in a Quantum World
Abstract

We prove that quantum-hard one-way functions imply simulation-secure quantum oblivious transfer (QOT), which is known to suffice for secure computation of arbitrary quantum functionalities. Furthermore, our construction only makes black-box use of the quantum-hard one-way function.
Our primary technical contribution is a construction of extractable and equivocal quantum bit commitments based on the black-box use of quantum-hard one-way functions in the standard model. Instantiating the Crépeau-Kilian (FOCS 1988) framework with these commitments yields simulation-secure quantum oblivious transfer.

2021

EUROCRYPT

Post-Quantum Multi-Party Computation
Abstract

We initiate the study of multi-party computation for classical functionalities in the plain model, with security against malicious quantum adversaries. We observe that existing techniques readily give a polynomial-round protocol, but our main result is a construction of *constant-round* post-quantum multi-party computation. We assume mildly super-polynomial quantum hardness of learning with errors (LWE), and quantum polynomial hardness of an LWE-based circular security assumption.
Along the way, we develop the following cryptographic primitives that may be of independent interest:
1.) A spooky encryption scheme for relations computable by quantum circuits, from the quantum hardness of (a circular variant of) the LWE problem. This immediately yields the first quantum multi-key fully-homomorphic encryption scheme with classical keys.
2.) A constant-round post-quantum non-malleable commitment scheme, from the mildly super-polynomial quantum hardness of LWE.
To prove the security of our protocol, we develop a new straight-line non-black-box simulation technique against parallel sessions that does not clone the adversary's state. This technique may also be relevant to the classical setting.

2021

EUROCRYPT

Black-Box Non-Interactive Non-Malleable Commitments
Abstract

There has been recent exciting progress in building non-interactive non-malleable commitments from judicious assumptions. All proposed approaches proceed in two steps. First, obtain simple “base” commitment schemes for very small tag/identity spaces based on a various sub-exponential hardness assumptions. Next, assuming sub-exponential non-interactive witness indistinguishable proofs (NIWIs), and variants of keyless collision-resistant hash functions, construct non-interactive compilers that convert tag-based non-malleable commitments for a small tag space into tag-based non-malleable commitments for a larger tag space.
We propose the first black-box construction of non-interactive non-malleable commitments. Our key technical contribution is a novel implementation of the non-interactive proof of consistency required for tag amplification. Prior to our work, the only known approach to tag amplification without setup and with black-box use of the base scheme (Goyal, Lee, Ostrovsky, and Visconti, FOCS 2012) added multiple rounds of interaction.
Our construction satisfies the strongest known definition of non-malleability, i.e., CCA (chosen commitment attack) security. In addition to being black-box, our approach dispenses with the need for sub-exponential NIWIs, that was common to all prior work. Instead of NIWIs, we rely on sub-exponential hinting PRGs which can be obtained based on a broad set of assumptions such as sub-exponential CDH or LWE.

2021

EUROCRYPT

Non-interactive Distributional Indistinguishability (NIDI) and Non-Malleable Commitments
Abstract

We introduce non-interactive distributionally indistinguishable arguments (NIDI) to remedy a significant weakness of NIWI proofs: namely, the lack of meaningful secrecy when proving statements about NP languages with unique witnesses.
NIDI arguments allow a prover $\cP$ to send a single message to verifier $\cV$, given which $\cV$ can obtain a sample $d$ from a (secret) distribution $\cD$ together with a proof of membership of $d$ in an NP language. The soundness guarantee is that if the sample $d$ obtained by the verifier $\cV$ is not in the language, then $\cV$ outputs $\bot$. The secrecy guarantee is that secrets about the distribution remain hidden: for every pair of (sufficiently) hard-to-distinguish distributions $\cD_0$ and $\cD_1$, a NIDI that outputs samples from $\cD_0$ with proofs is indistinguishable from one that outputs samples from $\cD_1$ with proofs.
We build NIDI arguments that satisfy secrecy for sufficiently hard distributions, assuming sub-exponential indistinguishability obfuscation and sub-exponentially secure (variants of) one-way functions. We demonstrate preliminary applications of NIDI and of our techniques to obtaining the first (relaxed) non-interactive constructions in the plain model, from well-founded assumptions, of:
-- Commit-and-prove that provably hides the committed message
-- CCA-secure commitments against non-uniform adversaries.
The commit phase of our commitment schemes consists of a single message from the committer to the receiver, followed by a randomized output by the receiver (that need not be sent to the committer).

2021

PKC

On the CCA Compatibility of Public-Key Infrastructure
📺
Abstract

In this work, we put forth the notion of compatibility of any key generation or setup algorithm. We focus on the specific case of encryption, and say that a key generation algorithm KeyGen is X-compatible (for X \in {CPA, CCA1, CCA2}) if there exist encryption and decryption algorithms that together with KeyGen, result in an X-secure public-key encryption scheme.
We study the following question: Is every CPA-compatible key generation algorithm also CCA-compatible? We obtain the following answers:
- Every sub-exponentially CPA-compatible KeyGen algorithm is CCA1-compatible, assuming the existence of hinting PRGs and sub-exponentially secure keyless collision resistant hash functions.
- Every sub-exponentially CPA-compatible KeyGen algorithm is also CCA2-compatible, assuming the existence of non-interactive CCA2 secure commitments, in addition to sub-exponential security of the assumptions listed in the previous bullet.
Here, sub-exponentially CPA-compatible KeyGen refers to any key generation algorithm for which there exist encryption and decryption algorithms that result in a CPA-secure public-key encryption scheme {\em against sub-exponential adversaries}.
This gives a way to perform CCA secure encryption given any public key infrastructure that has been established with only (sub-exponential) CPA security in mind. The resulting CCA encryption makes black-box use of the CPA scheme and all other underlying primitives.

2021

CRYPTO

Improved Computational Extractors and their Applications
Abstract

Recent exciting breakthroughs have achieved the first two-source extractors that operate in the low min-entropy regime. Unfortunately, these constructions suffer from non-negligible error, and reducing the error to negligible remains an important open problem. In recent work, Garg, Kalai, and Khurana (GKK, Eurocrypt 2020) investigated a meaningful relaxation of this problem to the computational setting, in the presence of a common random string (CRS). In this relaxed model, their work built explicit two-source extractors for a restricted class of unbalanced sources with min-entropy n^{\gamma} (for some constant \gamma) and negligible error, under the sub-exponential DDH assumption.
In this work, we investigate whether computational extractors in the CRS model be applied to more challenging environments. Specifically, we study network extractor protocols (Kalai et al., FOCS 2008) and extractors for adversarial sources (Chattopadhyay et al., STOC 2020) in the CRS model. We observe that these settings require extractors that work well for balanced sources, making the GKK results inapplicable.
We remedy this situation by obtaining the following results, all of which are in the CRS model and assume the sub-exponential hardness of DDH.
- We obtain ``optimal'' computational two-source and non-malleable extractors for balanced sources: requiring both sources to have only poly-logarithmic min-entropy, and achieving negligible error. To obtain this result, we perform a tighter and arguably simpler analysis of the GKK extractor.
- We obtain a single-round network extractor protocol for poly-logarithmic min-entropy sources that tolerates an optimal number of adversarial corruptions. Prior work in the information-theoretic setting required sources with high min-entropy rates, and in the computational setting had round complexity that grew with the number of parties, required sources with linear min-entropy, and relied on exponential hardness (albeit without a CRS).
- We obtain an ``optimal'' adversarial source extractor for poly-logarithmic min-entropy sources, where the number of honest sources is only 2 and each corrupted source can depend on either one of the honest sources. Prior work in the information-theoretic setting had to assume a large number of honest sources.

2021

CRYPTO

On the Round Complexity of Secure Quantum Computation
Abstract

We construct the first constant-round protocols for secure quantum computation in the two-party (2PQC) and multi-party (MPQC) settings with security against malicious adversaries. Our protocols are in the common random string (CRS) model.
- Assuming two-message oblivious transfer (OT), we obtain (i) three-message 2PQC, and (ii) five-round MPQC with only three rounds of online (input-dependent) communication; such OT is known from quantum-hard Learning with Errors (QLWE). - Assuming sub-exponential hardness of QLWE, we obtain (i) three-round 2PQC with two online rounds and (ii) four-round MPQC with two online rounds. - When only one (out of two) parties receives output, we achieve minimal interaction (two messages) from two-message OT; classically, such protocols are known as non-interactive secure computation (NISC), and our result constitutes the first maliciously-secure quantum NISC. Additionally assuming reusable malicious designated-verifier NIZK arguments for NP (MDV-NIZKs), we give the first MDV-NIZK for QMA that only requires one copy of the quantum witness. Finally, we perform a preliminary investigation into two-round secure quantum computation where each party must obtain output. On the negative side, we identify a broad class of simulation strategies that suffice for classical two-round secure computation that are unlikely to work in the quantum setting. Next, as a proof-of-concept, we show that two-round secure quantum computation exists with respect to a quantum oracle.

2021

CRYPTO

On the Round Complexity of Black-Box Secure MPC
Abstract

We consider the question of minimizing the round complexity of secure multiparty computation (MPC) protocols that make a black-box use of simple cryptographic primitives in the setting of security against any number of malicious parties. In the plain model, previous black-box protocols required a high constant number of rounds (>15). This is far from the known lower bound of 4 rounds for protocols with black-box simulators.
When allowing a random oblivious transfer (OT) correlation setup, 2-round protocols making a black-box use of a pseudorandom generator were previously known. However, such protocols were obtained via a round-collapsing ``protocol garbling'' technique that has poor concrete efficiency and makes a non-black-box use of an underlying malicious-secure protocol.
We improve this state of affairs by presenting the following types of black-box protocols.
a. 4-round ``pairwise MPC'' in the plain model.
This round-optimal protocol enables each ordered pair of parties to compute a function of both inputs whose output is delivered to the second party. The protocol makes black-box use of any public-key encryption (PKE) with pseudorandom public keys. As a special case, we get a black-box round-optimal realization of secure (copies of) OT between every ordered pair of parties.
b. 2-round MPC from OT correlations.
This round-optimal protocol makes a black-box use of any general 2-round MPC protocol satisfying an augmented notion of semi-honest security. In the two-party case, this yields new kinds of 2-round black-box protocols.
c. 5-round MPC in the plain model.
This protocol makes a black-box use of PKE with pseudorandom public keys, and 2-round oblivious transfer with ``semi-malicious'' security.
A key technical tool for the first result is a novel combination of split-state non-malleable codes (Dziembowski, Pietrzak, and Wichs, JACM '18) with standalone secure {\em two-party} protocols. The second result is based on a new round-optimized variant of the ``IPS compiler'' (Ishai, Prabhakaran and Sahai, Crypto '08). The third result is obtained via a specialized combination of these two techniques.

2021

CRYPTO

Compact Ring Signatures from Learning With Errors
Abstract

Ring signatures allow a user to sign a message on behalf of a ``ring'' of signers, while hiding the true identity of the signer. As the degree of anonymity guaranteed by a ring signature is directly proportional to the size of the ring, an important goal in cryptography is to study constructions that minimize the size of the signature as a function of the number of ring members.
In this work, we present the first compact ring signature scheme (i.e., where the size of the signature grows logarithmically with the size of the ring) from the (plain) learning with errors (LWE) problem. The construction is in the standard model and it does not rely on a trusted setup or on the random oracle heuristic. In contrast with the prior work of Backes
\etal~[EUROCRYPT'2019], our scheme does not rely on bilinear pairings, which allows us to show that the scheme is post-quantum secure assuming the quantum hardness of LWE.
At the heart of our scheme is a new construction of compact and statistically witness-indistinguishable ZAP arguments for NP $\cap$ coNP, that we show to be sound based on the plain LWE assumption. Prior to our work, statistical ZAPs (for all of NP) were known to exist only assuming \emph{sub-exponential} LWE. We believe that this scheme might find further applications in the future.

2020

EUROCRYPT

Statistical ZAP Arguments
📺
Abstract

Dwork and Naor (FOCS'00) first introduced and constructed two message public coin witness indistinguishable proofs (ZAPs) for NP based on trapdoor permutations. Since then, ZAPs have also been obtained based on the decisional linear assumption on bilinear maps, and indistinguishability obfuscation, and have proven extremely useful in the design of several cryptographic primitives.
However, all known constructions of two-message public coin (or even publicly verifiable) proof systems only guarantee witness indistinguishability against computationally bounded verifiers.
In this paper, we construct the first public coin two message witness indistinguishable (WI) arguments for NP with {\em statistical} privacy, assuming quasi-polynomial hardness of the learning with errors (LWE) assumption. We also show that the same protocol has a super-polynomial simulator (SPS), which yields the first public-coin SPS statistical zero knowledge argument.
Prior to this, there were no known constructions of two-message publicly verifiable WI protocols under lattice assumptions, even satisfying the weaker notion of computational witness indistinguishability.

2020

EUROCRYPT

Low Error Efficient Computational Extractors in the CRS Model
📺
Abstract

In recent years, there has been exciting progress on building two-source extractors for sources with low min-entropy. Unfortunately, all known explicit constructions of two-source extractors in the low entropy regime suffer from non-negligible error, and building such extractors with negligible error remains an open problem. We investigate this problem in the computational setting, and obtain the following results.
We construct an explicit 2-source extractor, and even an explicit non-malleable extractor, with negligible error, for sources with low min-entropy, under computational assumptions in the Common Random String (CRS) model. More specifically, we assume that a CRS is generated once and for all, and allow the min-entropy sources to depend on the CRS. We obtain our constructions by using the following transformations.
- Building on the technique of [BHK11], we show a general transformation for converting any computational 2-source extractor (in the CRS model) into a computational non-malleable extractor (in the CRS model), for sources with similar min-entropy.
We emphasize that the resulting computational non-malleable extractor is resilient to arbitrarily many tampering attacks (a property that is impossible to achieve information theoretically). This may be of independent interest.
This transformation uses cryptography, and in particular relies on the sub-exponential hardness of the Decisional Diffie Hellman (DDH) assumption.
- Next, using the blueprint of [BACD+17], we give a transformation converting our computational non-malleable extractor (in the CRS model) into a computational 2-source extractor for sources with low min-entropy (in the CRS model). Our 2-source extractor works for unbalanced sources: specifically, we require one of the sources to be larger than a specific polynomial in the other.
This transformation does not incur any additional assumptions. Our analysis makes a novel use of the leakage lemma of Gentry and Wichs [GW11].

2020

TCC

On Statistical Security in Two-Party Computation
📺
Abstract

There has been a large body of work characterizing the round complexity of general-purpose maliciously secure two-party computation (2PC) against probabilistic polynomial time adversaries. This is particularly true for zero-knowledge, which is a special case of 2PC. In fact, in the special case of zero knowledge, optimal protocols with unconditional security against one of the two players have also been meticulously studied and constructed.
On the other hand, general-purpose maliciously secure 2PC with statistical or unconditional security against one of the two participants, has remained largely unexplored so far. In this work, we initiate the study of such protocols, which we refer to as 2PC with one-sided statistical security. We completely settle the round complexity of 2PC with one-sided statistical security with respect to black-box simulation by obtaining the following tight results:
- In a setting where only one party obtains an output, we design 2PC in 4 rounds with statistical security against receivers and computational security against senders.
- In a setting where both parties obtain outputs, we design 2PC in 5 rounds with computational security against the party that obtains output first and statistical security against the party that obtains output last.
Katz and Ostrovsky (CRYPTO 2004) showed that 2PC with black-box simulation requires at least 4 rounds when one party obtains an output and 5 rounds when both parties obtain outputs, even when only computational security is desired against both parties. Thus in these settings, not only are our results tight, but they also show that statistical security is achievable at no extra cost to round complexity.
This still leaves open the question of whether 2PC can be achieved with black-box simulation in 4 rounds with statistical security against senders and computational security against receivers. Based on a lower bound on computational zero-knowledge proofs due to Katz (TCC 2008), we observe that the answer is negative unless the polynomial hierarchy collapses.

2019

CRYPTO

Non-interactive Non-malleability from Quantum Supremacy
📺
Abstract

We construct non-interactive non-malleable commitments without setup in the plain model, under well-studied assumptions.First, we construct non-interactive non-malleable commitments w.r.t. commitment for $$\epsilon \log \log n$$ tags for a small constant $$\epsilon > 0$$, under the following assumptions:1.Sub-exponential hardness of factoring or discrete log.2.Quantum sub-exponential hardness of learning with errors (LWE).
Second, as our key technical contribution, we introduce a new tag amplification technique. We show how to convert any non-interactive non-malleable commitment w.r.t. commitment for $$\epsilon \log \log n$$ tags (for any constant $$\epsilon >0$$) into a non-interactive non-malleable commitment w.r.t. replacement for $$2^n$$ tags. This part only assumes the existence of sub-exponentially secure non-interactive witness indistinguishable (NIWI) proofs, which can be based on sub-exponential security of the decisional linear assumption.Interestingly, for the tag amplification technique, we crucially rely on the leakage lemma due to Gentry and Wichs (STOC 2011). For the construction of non-malleable commitments for $$\epsilon \log \log n$$ tags, we rely on quantum supremacy. This use of quantum supremacy in classical cryptography is novel, and we believe it will have future applications. We provide one such application to two-message witness indistinguishable (WI) arguments from (quantum) polynomial hardness assumptions.

2018

CRYPTO

Promise Zero Knowledge and Its Applications to Round Optimal MPC
📺
Abstract

We devise a new partitioned simulation technique for MPC where the simulator uses different strategies for simulating the view of aborting adversaries and non-aborting adversaries. The protagonist of this technique is a new notion of promise zero knowledge (ZK) where the ZK property only holds against non-aborting verifiers. We show how to realize promise ZK in three rounds in the simultaneous-message model assuming polynomially hard DDH (or QR or N$$^{th}$$-Residuosity).We demonstrate the following applications of our new technique:We construct the first round-optimal (i.e., four round) MPC protocol for general functions based on polynomially hard DDH (or QR or N$$^{th}$$-Residuosity).We further show how to overcome the four-round barrier for MPC by constructing a three-round protocol for “list coin-tossing” – a slight relaxation of coin-tossing that suffices for most conceivable applications – based on polynomially hard DDH (or QR or N$$^{th}$$-Residuosity). This result generalizes to randomized input-less functionalities.
Previously, four round MPC protocols required sub-exponential-time hardness assumptions and no multi-party three-round protocols were known for any relaxed security notions with polynomial-time simulation against malicious adversaries.In order to base security on polynomial-time standard assumptions, we also rely upon a leveled rewinding security technique that can be viewed as a polynomial-time alternative to leveled complexity leveraging for achieving “non-malleability” across different primitives.

2018

TCC

Round Optimal Black-Box “Commit-and-Prove”
Abstract

Motivated by theoretical and practical considerations, an important line of research is to design secure computation protocols that only make black-box use of cryptography. An important component in nearly all the black-box secure computation constructions is a black-box commit-and-prove protocol. A commit-and-prove protocol allows a prover to commit to a value and prove a statement about this value while guaranteeing that the committed value remains hidden. A black-box commit-and-prove protocol implements this functionality while only making black-box use of cryptography.In this paper, we build several tools that enable constructions of round-optimal, black-box commit and prove protocols. In particular, assuming injective one-way functions, we design the first round-optimal, black-box commit-and-prove arguments of knowledge satisfying strong privacy against malicious verifiers, namely:Zero-knowledge in four rounds and,Witness indistinguishability in three rounds.
Prior to our work, the best known black-box protocols achieving commit-and-prove required more rounds.We additionally ensure that our protocols can be used, if needed, in the delayed-input setting, where the statement to be proven is decided only towards the end of the interaction. We also observe simple applications of our protocols towards achieving black-box four-round constructions of extractable and equivocal commitments.We believe that our protocols will provide a useful tool enabling several new constructions and easy round-efficient conversions from non-black-box to black-box protocols in the future.

2018

TCC

Upgrading to Functional Encryption
Abstract

The notion of Functional Encryption (FE) has recently emerged as a strong primitive with several exciting applications. In this work, we initiate the study of the following question: Can existing public key encryption schemes be “upgraded” to Functional Encryption schemes without changing their public keys or the encryption algorithm? We call a public-key encryption scheme with this property to be FE-compatible. Indeed, assuming ideal obfuscation, it is easy to see that every CCA-secure public-key encryption scheme is FE-compatible. Despite the recent success in using indistinguishability obfuscation to replace ideal obfuscation for many applications, we show that this phenomenon most likely will not apply here. We show that assuming fully homomorphic encryption and the learning with errors (LWE) assumption, there exists a CCA-secure encryption scheme that is provably not FE-compatible. We also show that a large class of natural CCA-secure encryption schemes proven secure in the random oracle model are not FE-compatible in the random oracle model.Nevertheless, we identify a key structure that, if present, is sufficient to provide FE-compatibility. Specifically, we show that assuming sub-exponentially secure iO and sub-exponentially secure one way functions, there exists a class of public key encryption schemes which we call Special-CCA secure encryption schemes that are in fact, FE-compatible. In particular, each of the following popular CCA secure encryption schemes (some of which existed even before the notion of FE was introduced) fall into the class of Special-CCA secure encryption schemes and are thus FE-compatible:1.[CHK04] when instantiated with the IBE scheme of [BB04].2.[CHK04] when instantiated with any Hierarchical IBE scheme.3.[PW08] when instantiated with any Lossy Trapdoor Function.

#### Program Committees

- TCC 2020
- Eurocrypt 2019

#### Coauthors

- Amit Agarwal (1)
- Saikrishna Badrinarayanan (5)
- James Bartusek (3)
- Rahul Chatterjee (1)
- Andrea Coladangelo (2)
- Rex Fernando (1)
- Rachit Garg (1)
- Sanjam Garg (1)
- Ankit Garg (1)
- Vipul Goyal (3)
- Mohammad Hajiabadi (1)
- Dennis Hofheinz (1)
- Yuval Ishai (1)
- Tibor Jager (1)
- Abhishek Jain (3)
- Aayush Jain (1)
- Yael Tauman Kalai (5)
- Daniel Kraschewski (1)
- Xiaohui Liang (1)
- George Lu (1)
- Fermi Ma (2)
- Hemanta K. Maji (3)
- Giulio Malavolta (2)
- Muhammad Haris Mughees (1)
- Rafail Ostrovsky (2)
- Omkant Pandey (1)
- Manoj Prabhakaran (1)
- Vanishree Rao (1)
- Ron D. Rothblum (1)
- Amit Sahai (12)
- Sina Shiehian (1)
- Akshayaram Srinivasan (3)
- Ivan Visconti (1)
- Brent Waters (5)
- Mark Zhandry (1)